Noise generated by aircraft jet engines during takeoff and landing is a matter of serious concern in most metropolitan areas of the world. In the United States alone, approximately five million people live or work adjacent to airports and are affected significantly by aircraft noise. Many municipalities have taken action to require reduction in aircraft noise. Much work has been done on designing turbofan aircraft engines to reduce noise levels. For background information relating to noise reduction systems for jet engines, reference may be had to the following U.S. Pat. Nos. 3,710,890; 4,077,206; 4, 117,671; 4,501,393; 4,909,346; 5,060,471; 5,127,602 and 5,167,118. These patents are incorporated herein by reference.
It generally can be said that in turbofan jet aircraft engines, the engine airflow is split into two parts as it passes through the engine, i.e. the primary or core flow and the fan or bypass flow. The primary or core flow passes through the low pressure and high pressure compressors and into the combustion chamber where fuel is mixed with the high pressure air and burned. The core flow then passes through the high and low pressure turbines and into the exhaust duct. The fan or bypass air flow only passes through the fan and is routed around the core engine and into the exhaust duct. In low bypass ratio confluent turbofan nacelles, the two flows enter into the exhaust duct at approximately equal pressure but at much different temperatures (approximately 230.degree. F. for the bypass flow and approximately 1100.degree. F. for the core flow). Unless mixed, the two flows remain substantially separate as they exhaust through the tailpipe of the jet engine. The hot core flow exits the engine at very high velocity which is much greater than the velocity of the fan gas. This hot core gas generates a large part of the engine jet noise.
One apparatus which has been developed and which has achieved beneficial results in the noise reduction of turbofan aircraft engines is the flow mixer of the multi-channel or multi-lobe inverted flow type. Examples of use of such mixers for noise suppression are found in the previously referenced U.S. Pat. Nos. 4,117,671 and 4,077,206. These flow mixers mix the two gas flows to more or less cause all the gas flowing through the tailpipe to flow at the same velocity. These flow mixers have been credited with noise reduction in the range of 3.5 to 4.5 decibels (Db) in the Effective Perceived Noise Level (EPNL), depending upon the engine cycle and bypass ratio. While this noise reduction is helpful, it is not sufficient within itself to solve all of the low bypass turbofan engine noise problems, and for this reason the exhaust flow mixer has had a limited commercial application.
To obtain larger magnitudes of noise reduction in jet engines, a traditional approach has been to mix ambient air flow with the jet engine flow to reduce jet velocity and associated noise. (Attention is called to U.S. Pat. Nos. 3,710,890 supra.) In order to provide large noise reduction, large ejector inlets with high secondary air flows have been used which have resulted in unacceptable levels of net thrust loss at cruise speeds.
The present disclosure provides a fixed geometry noise suppression system using ambient air flow in a manner that substantially eliminates the thrust sacrifices experienced with previous designs.